This invention relates to a novel class of solvent release coats for pressure-sensitive tapes, and, more specifically, to poly ketone resin -- release agent blends. In addition to their excellent release properties, these coatings were found to exhibit outstanding adhesion to impervious plastic film substrates as well as to commercially available flexographic inks. Briefly, we have discovered a multifunctional coating which offers to the pressure-sensitive tape industry an unusual combination of desirable properties, versatility, and marketing potential at relatively low cost.
Patents have described a variety of pressure-sensitive tapes comprising in essence a normally tacky and pressure-sensitive, elastomeric adhesive, a flexible backing or substrate and a release coat which facilitates the unwinding of tapes made in roll form. For the most common pressure-sensitive tapes with various substrates, satisfactory unwind properties are generally achieved with a release coating comprising stearates, metallic Werner complex compounds, or polymers with a carbon chain back-bone having attached thereto an adequate number of fatty acid radicals represented by alkyl groups with from 10 - 22 carbon atoms. Of equal importance for practical purposes is the presence of polar groups (hydroxy, carboxy, amino, etc.) in the polymeric back-bone or structure of the commercial release agents. We theorize that their primary function is to insure proper adhesion to various tape backings and thus prevent partial stripping or even total transfer to the adjacent adhesive layer. Although very efficient at relatively low coating weights, the above standard release agents such as, for example, "Quilon", the Werner complex compound supplied by du Pont or the release polymers described in U.S. Pat. Nos. 2,532,011; 2,816,655; 3,475,196, exhibited very poor ink receptivity thus precluding any attempts to print the respective tape backings altogether. Another deficiency of standard release agents is their limited ability to adhere strongly to non-polar substrates such as the great variety of plastic films utilized throughout the industry. When printability is a desired feature, this limitation is even more critical since strong anchorage to the plastic film substrates is a crucial requirement.
Numerous attempts have been made in the past to overcome the above-mentioned limitations of prior release agents. Thus, a logical approach would involve the synthesis of a polymeric back-bone comprising an increased number of polar groups capable of providing additional anchoring sites. However, due to the delicate balance of hydrophobic and hydrophilic groups, this would lead to decreased release efficiency which is not desirable. Another approach involving surface roughening and subsequent application of a pigmented backsize coating was found too cumbersome and produced an objectionable haziness, thus precluding the production of transparent tapes. In the case of pressure-sensitive cellophane tapes, the printability problem was solved by a different approach; namely, printing of the rather absorptive cellophane film prior to the tape manufacturing process. According to another method, the release agent is blended with vinyl chloride/vinyl acetate copolymers or other film formers to provide a release coating composition capable of filling in the rugosities of the backing or otherwise modify the release material or the properties of the backing.
Subsequent attempts to utilize similar or related formulations, including those described in U.S. Pat. No. 3,543,920 as printable release coats for plastic film substrates, were found marginal in performance for one or several of the following reasons:
1. Poor adhesion to a variety of impervious plastic films.
2. Haziness of dried coatings.
3. Borderline ink anchorage resulting in pickoff by pressure-sensitive adhesives.
4. Detackification tendency of adjacent adhesive at the very low coating weights applied on common film tapes.